Re: [PATCH v3 3/3] iova: defer maple tree erase on GFP_ATOMIC failure

[Ashok Raj]

On Tue, Jun 02, 2026 at 11:35:48PM -0400, Rik van Riel wrote:
> From: Rik van Riel <riel@xxxxxxxx>
> 

Hi Rik,

I'm a bit rust on these, looking a them after a very long time. So take
my question with a grain of salt :-)


[snip]

> +static void iova_deferred_free_work(struct work_struct *work)
> +{
> +	struct delayed_work *dwork = to_delayed_work(work);
> +	struct iova_domain *iovad = container_of(dwork, struct iova_domain,
> +						 deferred_free_work);
> +	struct llist_node *list = llist_del_all(&iovad->deferred_frees);
> +	struct llist_node *node, *next;
> +
> +	llist_for_each_safe(node, next, list) {
> +		struct iova *iova = container_of(node, struct iova,
> +						 deferred_free);
> +		unsigned long flags;
> +
> +		spin_lock_irqsave(&iovad->iova_lock, flags);
> +		if (remove_iova(iovad, iova))
> +			free_iova_mem(iova);
> +		else
> +			llist_add(&iova->deferred_free,
> +				  &iovad->deferred_frees);
> +		spin_unlock_irqrestore(&iovad->iova_lock, flags);
> +	}
> +
> +	if (!llist_empty(&iovad->deferred_frees))
> +		schedule_delayed_work(&iovad->deferred_free_work,
> +				      msecs_to_jiffies(10));

  If remove_iova() keeps failing (mas_store_gfp(NULL, GFP_ATOMIC) still
  can't get a maple tree node), the iova is pushed back onto deferred_frees
  and the work reschedules itself at a flat 10ms — forever.  There is no
  retry counter or no backoff.

  Under sustained memory pressure a device doing frequent unmap could
  accumulate a growing deferred list, exhaust its IOVA space, and start
  failing DMA map calls — with nothing in dmesg pointing at the real cause.

  Should we:

  1. A simple retry counter per iova (or a domain-level counter) with a
     WARN_ONCE after, say, 10 consecutive failures would at least make the
     condition visible.

  2. Exponential backoff (10ms → 50ms → 200ms, capped at e.g. 1s) reduces
     wasted wakeups under sustained pressure without delaying recovery when
     memory frees up quickly.

Cheers,
Ashok